1. Technical Field of the Invention
The present invention relates to microelectronics, and especially to integrated circuits comprising photodiodes.
2. Description of Related Art
Image sensors based on semiconductor components advantageously rely on the principle of photons being converted into electron/hole pairs in silicon. More precisely, the charges created in photosensitive regions are stored in the photodiode and are then read by an electronic system. This electronic system, which controls the photodiode, comprises in particular a read transistor for converting the charges stored in the photodiode into an electrical quantity.
The invention applies advantageously but without limitation to CMOS image sensors and more particularly to VMIS sensors (Vth modulation image sensors) which are image sensors that rely on the modulation of the threshold voltage of an MOS transistor. The reader may refer on this subject to the article by Takashi Miida et al., “A 1.5 Mpixel Imager with Localized Hole Modulation Method”, ISSCC Dig. Tech. Pap., pp. 42-43, July 2002.
This type of CMOS transistor consists of a buried photodiode associated with a single slightly modified MOS transistor, owing to the fact that its substrate is a floating substrate, that is to say its potential can only be reached via an electrode, for example. This floating substrate acts as a charge storage region during charge integration, that is to say when the incident light generates electron/hole pairs in the photosensitive regions.
More generally, the invention may apply to any photodiode of the floating substrate type.
Conventionally, photodiodes of the floating substrate type provide the isolation between the various pixels of an image sensor by producing a read transistor in which the gate is of circular type as opposed to rectangular type. This is because, to completely isolate the floating substrate, it is necessary to surround it with a semiconductor region of opposite type to it, while preventing the source and drain from being short-circuited. It is therefore imperative to use photodiodes having a gate with a circular structure, in which the buried layer of the photodiode is connected to the drain but the source is isolated from the buried layer by the gate of the read transistor that surrounds it.
However, devices possessing this type of structure with round-gate read transistors are very bulky.
Although CMOS image sensors in which a single read transistor is needed per pixel do make it possible for the area of the integrated circuit to be greatly reduced, the use of a transistor with a circular/round gate cancels out the saving made in terms of area. In this case, a pixel associated with a circuit comprising a photodiode and a circular-gate read transistor has as large a footprint as a pixel associated with a conventional circuit comprising a photodiode and four transistors.
There is a need to provide a solution to this problem.